Wood pellets are a type of wood fuel, generally made from wood residues or wood chips from roundwood. Wood residues are usually produced as a byproduct of sawmilling and other wood transformation activities. The pellets are extremely dense (approximately 42 pounds per cubic foot) and can be produced with a low moisture content (below 10%) that allows them to be burned with a very high combustion efficiency. Further, their regular geometry and small size allow for automatic feeding with very fine calibration. They can be fed to a burner by auger feeding or by pneumatic conveying.
Pellets are produced by compressing dried wood feedstock material which has first passed through a dryer and hammer mill to provide a uniform dough-like mass. This mass is fed to a press where it is squeezed through a die having holes of the size required (normally 6 mm diameter, sometimes 8 mm or larger). The high pressure of the press causes the temperature of the wood to increase greatly, and the lignin plastifies slightly forming a natural ‘glue’ that holds the pellet together as it cools.
Pellets conforming to normal practices in industry may have less than 10% water content, are uniform in density, have good structural strength, and low dust and ash content. Because the wood fibers are broken down by the hammer mill, there is virtually no difference in the finished pellets between different wood types. Pellets can be made from nearly any wood variety, provided the pellet press is equipped with good instrumentation, the differences in feed material can be compensated for in the press regulation. The energy content of wood pellets typically ranges from about 7,000 BTU/lb to 8,000 BTU/lb.
Pellets are used for industrial purposes to produce heat and/or electricity, by electric utilities, and by individuals in pellet stoves to produce heat for homes. Many large European utilities use wood pellets to generate electricity to comply with renewable energy requirements.
Emissions such as NOx, SOx and volatile organic compounds from pellet burning equipment, are, in general, very low in comparison to other forms of combustion heating, making this one of the less-polluting heating options available. One possible problem is emission of fine dust (particulate matter), especially in urban areas, when there is a high concentration of pellet heating systems, coal, or oil heating systems in close proximity. The PM2.5 emissions of older pellet stoves and boilers can be problematic in close quarters, especially in comparison to natural gas (or renewable biogas), though electrostatic precipitators and baghouse particle filters for pellet heaters have been developed and considerably reduce the problem when installed as standard. This potential problem can also be mitigated by producing pellets with low ash content, which should be less than 0.7% for residential pellets. This low ash content can be achieved by debarking the wood prior to pelletizing.
A fully automated stove or oven only requires the initial actions of loading the pellets into the stove and turning on the stove. The stove then automatically does the rest including: (a) automatically lighting, (b) automatically feeding the pellets into the flame with an auger, (c) automatically adjusting the rate to keep the room at a pre-set temperature with an electric thermostat, etc.
The combustion of sustainably managed and harvested wood or biomass is generally considered to represent one of the best practical, and readily available, means for sustainable use of low net-carbon combustion for economic sectors that require the use of combustion. The medium of wood or biomass pellets, as a form of energy transfer, provides a fungible, mass-producible, commerce-ready product capable of putting sustainable biomass combustion technology to work for the modern-day consumer. Furthermore, in many cases, wood/biomass pellets are less expensive and nearly as convenient as other fuel sources.
High-efficiency wood pellet stoves and boilers have been developed in recent years, offering combustion efficiencies of over 90%. Wood pellet boilers—having limited control over the rate and presence of combustion compared to liquid or gaseous-fired systems—are particularly well suited for hydronic systems due to the hydronic system's greater ability to store heat. In particular, pellet burners capable of being retrofitted to oil-burning boilers are predicted to be available on the market within the next several years.
One of the problems curbing the growth of pellet fuel use in the United States is that a common, industry wide standard has not been developed or accepted. Various users employ the use of different standards or requirements throughout the United States and there is no regulatory authority enforcing the use of any particular standard. In addition, many of the standards or requirements deployed in industry have been developed by the Pellet Fuel Institute, and are generally less strict than those of Europe. For example, United States consumers accept and use pellets that are manufactured such that exposure of the pellets to large volumes of water in may result in significant degrades in the material (i.e., the pellets turn into “mush”). Even in view of a lack of an emerging standard within the United States, many manufacturers attempt to developed pellets that comply with “some” standard in an effort to product a quality product. This effort by manufactures is largely fueled by that fact that the warranties offered for United States manufactured or imported combustion equipment typically do cover damage caused by pellets that do not conform with regulations.
The prices for pellets manufactured in the United States surged during the fossil fuel price inflation that occurred in the years of 2007 to 2008. Subsequent to this time, the prices of pellets in the United States have decreased significantly beginning late in the year of 2008 and early 2009. Generally, the price for pellets in the United States is lower on a per-BTU basis than that of most fossil fuels, excluding coal. However, the use of coal is not a highly favored heating option for many residential and commercial consumers due to several reasons. A few of these reasons include the frequent maintenance/tending requirements of the equipment by end users, the high carbon emissions, air pollution (often leading to nuisance complaints from neighbors and/or investigation by boards of health and environmental agencies), the mess generated by coal dust and/or improper storage, as well as potentially dangerous fly ash.
The high density of pellets also permits for compact storage and rational transport over long distances. Furthermore, the pellets can be conveniently blown from a tanker to a storage bunker or silo on a customer's premises. As the price of heating with fossil fuels increases, the market potential for pellet heating solutions expands A large number of models of pellet stoves, central heating furnaces and other heating appliances have been developed and marketed since about 1999. With the surge in the price of fossil fuels beginning in the year 2005, the demand for such equipment has increased all over Europe and a sizable industry is emerging.
Turpentine is a wood extractive found in most coniferous woods. During the pulping process, wood chips are heated and lignin is dissolved which drives turpentine into the liquid/vapor space within a digester. In the batch cooking process, the vapor space of the digester is vented or relieved. Typically in industry, the turpentine is recovered by condensing the vapors generated in the digester.